Correlation spectroscopy is an attractive technique for sensing and analysis applications because it combines the attributes of mechanical and optical simplicity, high selectivity, and good sensitivity. In correlation spectroscopy, the degree of correlation between the transmission of an unknown sample and a reference cell containing a known target compound, or chemical species, is determined over a fixed spectral range. For materials, such as gases, that are characterized by narrow absorption lines, the cross correlation between the transmission spectra of different compounds can usually be neglected, and a significant correlation will be observed only when the sample and reference cells contain the same chemical species. Therefore, excellent selectivity is obtained. Unfortunately, because of the need for many reference cells containing target compounds, conventional correlation spectrometers tend to be large and unwieldy, and construction of spectrometers capable of detecting a large number of chemical species is impractical.
In FIG. 1A is shown a schematic illustration of a conventional transmissive correlation spectrometer 10. Broadband light 12 from a light source 11 first passes through a sample cell 13 containing an unknown sample to provide a sample transmission spectrum 14. The sample transmission spectrum light 14 then passes through a reference cell 15 containing a known target compound (or mixture of target compounds) to provide a reference transmission spectrum 16. Thus, the reference cell 15 functions as a complex spectral filter whose transmission spectrum is perfectly correlated with that of the target compound. To measure the degree of correlation between the two spectra 14 and 16, the transmission spectrum 16 of the reference cell 15 can be modulated by a suitable modulation means 17 (e.g., pressure or electric-field modulation of the linewidth), and the output of a single-channel broadband photodetector 19 at the modulation frequency is recorded. If the spectra 14 and 16 of the sample 13 and the reference 15 are highly correlated (i.e., both the sample cell 13 and the reference cell 15 contain the same chemical species), a large fractional modulation of the transmitted intensity will be recorded by the detector 19. Conversely, when the target compound is not present in the sample cell 13, the fractional modulation will be small. The output of the detector 19 at the modulation frequency then provides a measure of the degree of correlation between the sample and the target compound. Measurement of the degree of correlation between the transmission spectrum emerging from the sample cell and the transmission spectrum of the reference cell thereby allows the concentration of the target compound in the unknown sample to be determined. See R. Goody, “Cross-correlating spectrometer,” J. Opt. Soc. Am. 58, 900 (1968).
For high selectivity and good sensitivity, it is important that the spectral range used in determining the cross correlation is large enough to encompass several absorption bands of the target compound. An important requirement of any modulation method is the maintenance of a constant spectrally integrated transmission of the reference cell to minimize any background modulation of the transmitted light. Further, the transmission spectra for many compounds of interest are characterized by narrow, isolated, absorption bands and thus are near unity for most wavelengths. For these compounds, a large background of unmodulated radiation will reach the detector, since the overall transmission will be modulated only in spectral regions where the reference absorbs and the magnitude of the fractional modulation of the transmitted light will be limited. Because the relatively sharp spectral lines absorb only a small fraction of the light in a broad wavelength band, the detector must respond to a small, modulated signal superimposed on a large DC background. This severely limits the detector's dynamic range and raises the noise level. To overcome this difficulty, the total spectrum of light that is allowed to reach the detector is usually limited by means of a bandpass filter 18 to one or more regions near important absorption bands.
In FIG. 1B is shown a schematic illustration of a holographic correlation spectrometer wherein the reference cell 15 is replaced with a diffractive optical element 25 that synthesizes a desired spectral transfer function corresponding to a target compound at a predetermined diffraction angle θd. The spectral transfer function is modulated in a suitable fashion by modulation means 27 and the modulated output of the broadband photodetector 19 is measured. The diffractive element 25 can operate in either a reflective (as shown) or a transmissive mode. In this configuration, the total power measured by the detector at the diffraction angle θd is determined by the cross correlation between the transmission spectrum 14 of the sample cell 13 and the diffraction efficiency spectrum 26 of the diffractive element 25 (at θd). If the synthetic reference spectrum of the diffractive element 25 closely corresponds to the absorption spectrum of the target compound of interest, then the effect of the diffractive element 25 is analogous to that arising from a reference cell 15 containing a physical target compound. See M. B. Sinclair et al., “Synthetic spectra: a tool for correlation spectroscopy,” Appl. Optics 36(15), 3342 (1997); and M. B. Sinclair et al., “Synthetic infrared spectra,” Optics Lett. 22(13), 1036 (1997).
The use of diffractive elements to produce synthetic reference spectra has several advantages over the use of reference cells containing real physical target compounds. First, the diffractive elements can be extremely compact, allowing many to be stored on a common substrate. Alternatively, a single programmable diffraction grating can be used to recreate the spectra of a large number of materials. Thus, a correlation spectrometer relying on synthetic spectra can easily be configured to analyze for many compounds, eliminating the need for different, bulky reference cells. Second, the diffractive elements can be designed using multivariate analysis to reproduce only a subset of the target spectrum. This is desirable in situations where interference from overlapping absorption of other non-target compounds is expected. Reproducing only those portions of the target spectrum that are free from chemical interferences will increase selectivity. Third, the diffracted light spectrum can be modulated in ways not possible with real reference materials. In the case of simple wavelength modulation, the magnitude of the modulation can be significantly larger than can be achieved with a real reference material. More complex forms of modulation can be used. In this way, both the sensitivity and the selectivity of the spectrometer can be optimized for a given application. As described above, the spectrally integrated diffraction efficiency must remain constant to avoid spurious intensity modulation of the reflected optical spectrum. Fourth, diffractive elements can be designed to simulate materials that are difficult to handle (e.g., highly reactive, toxic, or caustic materials) or transient chemical species whose lifetimes are too short to allow their use as reference materials. Fifth, since there is spatial information in the direction perpendicular to the diffraction plane, an imaging correlation spectrometer can be developed by modifying a simple synthetic correlation spectrometer to include a detector array and push-broom collection optics. Finally, using dark-field correlation sensing (DFCS), the fractional modulation of the detected power (and, hence, the signal-to-noise ratio) can be larger when the reference spectrum is synthesized to be the complement of the transmission spectrum of the target compound. Then all wavelengths other than those matching absorbances of the target compound are blocked and only a small percentage of the incident light is transmitted. When the incoming light had spectral absorption lines matching the position of the programmed, modulated “transmission-complement” lines, a large modulation of the output occurs. However, now the modulation occurs against a relatively dark rather than bright background, with a manifold improvement in signal-to-background ratio and associated enhancement of the limit of detection, thereby reducing the difficultly of detecting a small AC signal on a large DC background by removing most of the DC background.
The production of synthetic spectra using diffractive elements requires that, at a fixed diffraction angle, the diffracted light spectrum accurately reproduces a desired spectrum. To maximize the intensity of the diffracted light spectrum, an element that imposes only a phase modulation on the incident light is preferable. Computer-generated diffractive optical elements can also be used to synthesize the infrared spectra of target compounds. These elements can be used to replace the reference cell in a conventional correlation spectrometer. A large number of such diffractive elements can be stored as phase gratings in a compact-disk-like format or a programmable diffractive grating can be used to enable the spectrometer to quickly characterize unknown samples.
However, the fabrication of such diffractive elements is difficult and the optical design of such holographic correlation spectrometers is complex, making miniaturization difficult. Therefore, a need remains for a simple correlation spectrometer that can be miniaturized.